Polymers having reactive terminal silyl groups or compositions comprising such polymers can be hydrolyzed and condensed in the presence of water and metal catalysts. Suitable known catalysts for curable compositions include compounds employing metals such as Sn, Ti, Zn, or Ca. Organotin compounds such as, for example, dibutyltin dilaurate (DBTDL) are widely used as condensation cure catalysts to accelerate the moisture-assisted curing of a number of different polyorganosiloxanes and non-silicone polymers having reactive terminal silyl groups such as room temperature vulcanizing (RTV) formulations including RTV-1 and RTV-2 formulations. Environmental regulatory agencies and directives, however, have increased or are expected to increase restrictions on the use of organotin compounds in formulated products. For example, while formulations with greater than 0.5 wt. % dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications during the next four to six years.
The use of alternative organotin compounds such as dioctyltin compounds and dimethyltin compounds can only be considered as a short-term remedial plan, as these organotin compounds may also be regulated in the future. It would be beneficial to identify non-tin-based accelerators that accelerate the condensation curing of moisture-curable silicones and non-silicones.
Substitutes for organotin catalysts should exhibit properties similar to organotin compounds in terms of curing, storage, and appearance. Non-tin accelerators would also desirably initiate the condensation reaction of the selected polymers and complete this reaction upon the surface and may be in the bulk in a desired time schedule. There are therefore many proposals for the replacement of organometallic tin compounds with other metal- and non-metal-based compounds. These new accelerators have specific advantages and disadvantages in view of replacing tin compounds perfectly. Therefore, there is still a need to address the weaknesses of possible non-tin compounds as suitable accelerators for condensation cure reactions. The physical properties of uncured and cured compositions also warrant examination, in particular to maintain the ability to adhere onto the surface of several substrates.
The use of non-metal-based accelerators in condensation curable compositions has been described. DE No. 3,411,716A1 claims the use of cyclic amidines as cure accelerators in moisture-curable siloxane compositions. U.S. Publication No. 2010/0004367 describes the use of amidines such as DBU, imidazoles, and alkyl- and aryl-substituted guanidines in ambient-temperature curable compositions. WO 2012/003160A1 claims the use of a base selected from amidines (preferably DBU), guanidines, and sulfur-containing bicyclic amidines, among others.
U.S. Pat. No. 4,769,412 discloses the use of guanidine compounds and guanidino group-containing silanes or siloxanes as possible choices for a curing catalyst in a room temperature-curable silicone rubber composition. U.S. Pat. No. 6,235,832 discloses guanidines, imidazoles, and amidines as a cure accelerator in a curable silicone composition. U.S. Publication No. 2010/0004367 claims a curable composition comprising an amidine compound which acts as the curing catalyst. U.S. Publication No. 2010/0063215 describes a curable composition which comprises an amidine compound and a sulfonyl-group-containing molecule. U.S. Publication No. 2010/0152373 claims the use of a Lewis acid and an amine compound in a curable composition, where the amine compound is described as an aryl-substituted guanidine compound and/or an aryl-substituted biguanide compound. EP No. 2,267,083A1 describes a curable composition using a guanidine compound in combination with a methyl-ester-group-containing compound.
U.S. Publication No. 2011/0046299 discloses the use of the metal-free organic catalyst based on a tetra-substituted, monosilylated guanidine as a condensation catalyst. U.S. Publication No. 2011/0098392 describes tetra-substituted guanidines that serve as condensation cure catalysts are free of silicon-containing substituents on the nitrogen atoms of the guanidine core. U.S. Publication No. 2011/028196 discloses a curable silicone composition using a penta-substituted guanidine as the catalyst. EP No. 2388297A1 discloses a non-organotin curable composition comprising an amidine compound, and more preferably a guanidine compound, and a chelating agent having a carbonyl group and/or a salt of the chelating agent. U.S. Publication No. 2011/020693 describes an article having antifouling properties which comprises at least one antifouling topcoat that is prepared from a curable polyorganosiloxane composition containing a silicon-containing guanidine.
The prior replacement accelerators for organotin compounds generally cannot maintain their ability to cure when exposed to humidity or ambient air after storage over months in a sealed cartridge. It is always a specific requirement for moisture-curable compositions to achieve the shortest possible curing times, showing a tack-free surface as well as curing through the complete bulk in thick section for RTV-1 and RTV-2 compositions. Additionally, such compositions should provide a reasonable adhesion after cure onto a variety of substrates. Thus, there is still a need for alternative materials to replace tin as a core accelerator in moisture curable compositions.